Process for regeneration of electrolyte containing tin salts by reducing the same

ABSTRACT

Process for reclaiming stannous ion by reducing the same in an electrolyte to stannic ion through a simple operation in a short period of time, which comprises the steps of preparing the electrolyte in an acidic range as required and then adding metallic tin to the electrolyte and heating the mixture to at least 80° C., in order to reduce the stannous ion oxidized in the electrolyte containing tin salt to obtain a stable electrolyte treated product. This process may be utilized particularly for effectively reducing and reclaiming an electrolyte which contains tin salt used for electrolytically coloring aluminum.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a process for the reduction regeneration of anelectro-plating bath or a coloring electrolyte for aluminum and moreparticularly to a process for the regeneration of an electrolyte byreducing stannic ions in the electrolyte into stannous ions through theaddition of metallic tin to the used or old electrolyte.

BACKGROUND OF THE INVENTION

A tin salt aqueous solution containing stannous ions (Sn²⁺) is generallyused as an electrolyte containing tin ions. This aqueous solutioncontaining stannous ions is very unstable since stannous ions are easilyexidized into stannic ions (Sn⁴⁺) by oxygen in the air or oxygengenerated during electrolysis.

As such stannic ions form in the electrolyte, the concentration ofstannous ions in the electrolyte decreases and the current efficiency ofelectrolysis is reduced. Furthermore, it gives rise to the seriousproblem that the stannic ions become insoluble substances such asstannic acid, etc. which are likely to become colloids and thereforegreat difficulty is encountered in removing such insoluble substances byfilteration. These insoluble substances attach onto the surface of theproduct and cause pitting thereon.

Therefore, in order to obtain good quality products, it is essential tocontrol the concentration of stannic ions in the electrolyte.

With regard to preventing insoluble precipitates from forming, it iseffective to add a chelating agent to the electrolyte to dissolve Sn⁴⁺.In this case, however, Sn⁴⁺ is accumulated in the electrolyte, leadingto an increase in the concentration of Sn⁴⁺ and, as a result,electrolytic coloring or electro-plating tend to become difficult.Therefore, a tin salt aqueous solution in which the concentration ofstannic ions has increased by oxidation has had to be abandoned.

The regeneration of the electrolyte by the reduction thereof has beeneffected. One of such regeneration methods is an electrolytic reductionmethod as described in Japanese Patent Publication No. 19856/1978. Inaccordance with this electrolytic reduction method of stannous ions andstannic ions which are present in the electrolyte, the stannous ions arefirst reduced into metallic tin and thereafter the stannic ions arereduced into metallic tin. Therefore, although this method is excellent,a very long processing time is required and equipment and operationcosts are increased.

Additionally, as the regeneration method of an electrolyte, a method ofreducing stannic ions into stannous ions by adding those metals whichare baser than tin has been carried out. According to this method,however, since the added metal is present in the electrolyte as a metalion, it may exert bad influences on electrolytic processing. It is,therefore, necessary to remove such metal ions, but the removaloperation of the metal ions is very complicated. Thus this method is notpreferred from practical and economic standpoints.

The inventors have studied to develop a method which removes thedisadvantages in the prior art as described above and which permits thereduction of stannic ions by a simplified procedure in a short time andthe formation of a regenerated electrolyte having a stable composition.As a result, it has been found that when metallic tin is added as areducing agent to the electrolyte, as necessary, after the pH of theelectrolyte has been controlled to the acidic region, the metallic tinand stannic ions undergo the oxidation-reduction reaction, promptly bothchanging into stannous ions.

DISCLOSURE OF THE INVENTION

This invention provides a process for the regeneration of an electrolytecontaining tin salts by reduction thereof which is characterized in thatin regenerating the electrolyte by reducing stannic ions therein intostannous ions, metallic tin is added to the electrolyte as it is orafter the pH of the electrolyte is controlled to the acidic region andthe resulting electrolyte is heated.

In accordance with the process of this invention, the stannic ions inthe electrolyte are regenerated into stannous ions. After the reactionis completed, the unreacted metallic tin is taken out of the electrolyteand, as necessary, additional processings such as adjustment in theconcentrations of other components in the electrolyte, adjustment in pH,etc. are applied. The thus regenerated electrolyte can be re-used.

When the above processings are continuously carried out, the electrolytecan be repeatedly used by recycling. Furthermore, in accordance with theprocess of this invention, the processing can be continuously carriedout and the processing time can be greatly shortened. For example, inthe method as described in Japanese Patent Publication No. 19856/1978,the time required for the reduction of the stannic ions is 6 hours ormore whereas in the process of this invention, it is 1 hour or less andusually sufficient to be about 30 minutes.

Additionally, equipment and operation costs can be lessened and thequality of the product obtained by using the regenerated electrolyte canbe stabilized.

PREFERRED EMBODIMENTS OF THE INVENTION

The electrolyte for use in this invention is subject to no speciallimitations so long as it contains tin salts. In a fresh electrolyte,almost all of the tin ions are stannous ions and there are almost nostannic ions. As the electrolysis proceeds, the concentration of stannicions increases whereas the concentration of stannous ions relativelydecreases.

The process of this invention is applied to an electrolyte in which theconcentration of stannic ions has increased to a certain extent, forexample, the ratio of Sn⁴⁺ /Sn²⁺ has reached 1/2 or more. In general,the electrolyte contains a chelating agent in order to keep the stannousand stannic ions in the state that they are dissolved.

Hereinafter the process of this invention will be explained in greaterdetail:

First, the pH of the electrolyte is controlled to the acidic region.This control is not always required when the pH of the electrolyte isalready in the acidic region. However, when it is weak acidic oralkaline it is necessary to lower the pH to the acidic region.

This control in the pH of the electrolyte is generally carried out byusing acids. Those acids releasing the same anions as those exisiting inthe electrolyte, such as sulfuric acid, hydrochloric acid, phosphoricacid, nitric acid, pyrophosphoric acid or various organic acids can beused singly or in admixtures comprising two or more thereof. In theprocess of this invention, sulfuric acid is most preferable.

The pH of the electrolyte is usually controlled to 4 or less andpreferably to the range of from 0.5 to 2. Where the pH is too low, thereduction rate of stannic ions is somewhat reduced although metallic tinis promptly dissolved. On the other hand, where the pH exceeds 4,metallic tin is less dissolved and the reduction does not proceedpromptly.

Thereafter, metallic tin is added to the electrolyte of which the pH hasbeen controlled to the acidic region and the resulting electrolyte isheated. In this case, it is preferred to add metallic tin having highactivity. For example, those having large specific surface areas, suchas fine powder, tin foils, sponges, etc. are preferably used.

Metallic tin having such a high specific surface area can be produced asfollows:

An acidic solution (e.g., an electro-plating solution or a coloringelectrolyte for aluminum) containing stannous ions and/or stannic ionsis used as an electrolyte and a direct current is passed through theelectrolyte with electrically conductive materials such as metal orcarbon as anode and cathode whereby tin ions (Sn²⁺, Sn⁴⁺) are depositedas metallic tin on the cathode. The thus obtained metallic tin is in asponge form, has a high specific surface area and can be easilyseparated from the cathode.

The amount of the metallic tin added to the electrolyte is not limitedand can be suitably determined depending upon the concentration ofstannic ions in the electrolyte, the required concentration of stannousions, etc. In general, it should be about 1 to 10 time moles of theamount required entirely to reduce stannic ions in the electrolyte. Evenif the metallic tin is excessively added, the unreacted metallic tinprecipitates in the bottom of the reactor and can easily be separated.Therefore, it imposes no specific hindrance onto the electrolyte and themetallic tin so separated can be re-used as it is.

In the process of this invention, it is required to heat the electrolyteto which the metallic tin has been added. The addition of metallic tincauses the oxidation-reduction reaction as illustrated below wherein themetallic tin and stannic ions react with each other, both formingstannous ions.

    Sn+Sn.sup.4+ →2Sn.sup.2+

Since the reaction is accelerated by heating, it is preferred from thestandpoint of acceleration of reaction to heat the electrolyte. However,where the electrolyte may be deteriorated in quality at hightemperatures, the electrolyte to which metallic tin is added should beprocessed at low temperatures. From such standpoint, the heatingtemperature is generally set in the range of from 80° C. to the boilingtemperature of the electrolyte.

With regard to the heating time, it is sufficient that the reductionreaction proceeds to the desired extent. While the heating time cannotbe determined unconditionally since it varies depending upon the shapeand amount of the metallic tin to be added and other various conditions,it is usually sufficient to be 1 hour or less.

The following examples are given to illustrate this invention in greaterdetail. In these examples, the concentration of stannic sulfate is shownas a concentration converted to SnSO₄.

EXAMPLE 1

(1) Production of Metallic Tin

A direct current was passed through an old or used coloring electrolytefor aluminum consisting of 9.0 grams/liter of stannous sulfate, 11.1grams/liter of stannic sulfate, 70 grams/liter of citric acid and 20grams/liter of sulfuric acid and having a pH of 1.5 with tin andstainless steel as anode and cathode respectively to depositesponge-like metallic tin on the cathode.

(2) Regeneration by Reducing

To an old or used coloring electrolyte for aluminum consisting of 4.0grams/liter of stannous sulfate, 15.1 grams/liter of stannic sulfate, 30grams/liter of tartaric acid, 30 grams/liter of nickel sulfate and 20grams/liter of ammonium sulfate and having a pH of 7.5 was added 40grams/liter of sulfuric acid to adjust the pH to 0.9. Thereafter, 18grams/liter of the sponge-like metallic tin as obtained in (1) was addedto the above electrolyte and the resulting mixture was heated for 20minutes at 100° C. After heating, the obtained electrolyte contained21.2 grams/liter of stannous sulfate and 0.7 gram/liter of stannicsulfate.

EXAMPLE 2

To an old or used neutral tin electro-plating solution consisting of 41grams/liter of stannous sulfate, 89 grams/liter of stannic sulfate, 150grams/liter of ammonium citrate and 100 grams/liter of ammonium sulfateand having a pH of 6.5 was added 60 grams/liter of sulfuric acid toadjust the pH to 0.7. Thereafter, 148 grams/liter of the sponge-likemetallic tin as obtained in Example 1 was added to the plating solutionand the resulting mixture was heated at 96° C. for 40 minutes. Afterheating, the obtained plating solution contained 222 grams/liter ofstannous sulfate and 8 grams/liter of stannic sulfate.

EXAMPLE 3

To an old or used lustrous tin-cobalt electro-plating solutionconsisting of 10 grams/liter of stannous sulfate, 10 grams/liter ofstannic sulfate, 50 grams/liter of cobalt sulfate and 200 grams/liter ofsodium pyrophosphate was added 60 grams/liter of sulfuric acid.Thereafter, 10.7 grams/liter of the sponge-like metallic tin as obtainedin Example 1 was added to the plating solution and the resulting mixturewas heated at 100° C. for 10 minutes. After heating, the obtainedplating solution contained 21 grams/liter of stannous sulfate and 4grams/liter of stannic sulfate.

EXAMPLE 4

To an old or used coloring electrolyte for aluminum consisting of 9.0grams/liter of stannous sulfate, 11 grams/liter of stannic sulfate, 70grams/liter of citric acid and 20 grams/liter of sulfuric acid andhaving a pH of 1.5 was added 21.4 grams/liter of the sponge-likemetallic tin as obtained in Example 1. The resulting mixture was heatedat 98° C. for 10 minutes. After heating, the obtained electrolytecontained 30 grams/liter of stannous sulfate and 4 grams/liter ofstannic sulfate.

EXAMPLE 5

The procedure of Example 4 was repeated except that 100 mesh powdery tinreagent on the market was used in place of the sponge-like metallic tin.After heating, the obtained electrolyte contained 11.5 grams/liter ofstannous sulfate and 9 grams/liter of stannic sulfate.

COMMERCIAL UTILIZATION

The process of this invention is greatly useful for the regeneration ofan electro-plating bath or a coloring electrolyte for aluminumcontaining tin salts.

We claim:
 1. A process for regenerating a stannous tin-containingelectrolyte which contains stannic ions comprisingif said electrolyte isnot acidic, adding sufficient acid to said electrolyte so that theelectrolyte becomes acidic; adding metallic tin having a high specificsurface area selected from the group consisting of powdered tin, thinfoils of tin, and sponge-like tin to said electrolyte; and heating saidelectrolyte containing said added metallic tin to from 80° C. to theboiling temperature of said electrolyte whereby stannic ions are reducedto stannous ions and metallic tin is oxidized to stannous ions therebyregenerating the electrolyte.
 2. The process of claim 1 wherein theelectrolyte to which the metallic tin is added has a pH of 4 or less. 3.The process of claim 1 wherein the electrolyte to which the metallic tinis added has a pH of from 0.5 to
 2. 4. The process of claim 3 whereinsulfuric acid is added to said electrolyte to adjust the pH to the rangeof from 0.5 to 2.